mm/migration: use helper macro min in do_pages_stat

mm/migration: use helper macro min in do_pages_stat

We could use helper macro min to help set the chunk_nr to simplify the
code.

Link: https://lkml.kernel.org/r/20220318111709.60311-7-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>